A variety of apparatus and methods e.g, Doppler radar sets, are known for the accurate measurement of the speed of motion of relatively fast moving objects. However, the measurement of slowly moving objects e.g., objects moving at a speed of a few inches per minute or being motionless, is a far more difficult problem. In the case of slow moving objects the Doppler frequency shift is very small; therefore, it is difficult to accurately determine absolute speed of an object and changes in speed of an object.
Measurement of the rate of rise of molten metal in bottom filled molds is an example of an environment in which the object moves slowly. Experience shows that accurate control of the rate of rise of the molten metal in th mold produces improved product. U.S. Pat. Nos. 4,019,562 and 4,642,777 are examples of prior art arrangements which utilize Doppler shifts to measure the rate of rise of molten metals. Bulletin No. 7642G, entitled Varactor Tuned Gunnplexer Transceiver "Front End", which is dated Apr. 1985 and issued by M/A COM Semiconductor Products Inc. describes commercially available Gunn oscillator transmitter/receivers and applications of those transceivers. A further undated publication of Microwave Associates entitled "Microwave Components for Motion Detection" describes further applications of such transceivers.